The present invention relates to a semiconductor laser module, and more particularly to an optical transmission module suitable for optical transmission systems.
In the optical communication system in which emitted light of a semiconductor laser is dynamically intensity modulated to be transmitted through an optical fiber, a high receiving sensitivity is required in order to attain ultra high-speed operation in the band of a transmission speed of the order of Gb/s and extend a transmissible distance without repeater. An optical signal being transmitted through the optical fiber causes an inter-symbol interference due to deterioration of a signal waveform, which is a main cause for deterioration of the receiving sensitivity.
Heretofore, in order to reduce the deterioration of the receiving sensitivity, a semiconductor laser having an excellent narrow spectral line width of a single mode is used. Further, there has been proposed in the collected papers in the national convention of the Institute of Electronics, Information and Communication Engineers of Japan, 1989, spring, pp. 4-100 that an impedance matching circuit is connected between a semiconductor laser and a driver circuit for driving the semiconductor laser in series to the semiconductor laser to thereby suppress a parasitic reactance component between semiconductor laser and the driver circuit so that a jitter due to the impedance mismatching is reduced to improve the transmission quality.
FIG. 2 is a block diagram schematically illustrating a conventional optical transmitter including an impedance matching circuit. An impedance matching circuit 100 is connected between a semiconductor laser 8 and a semiconductor laser driver circuit 1 for driving the laser 8. Connected to the semiconductor laser driver circuit is an input terminal for inputting a signal. The optical transmitter further includes a thermistor 4 for measuring a temperature of the semiconductor laser 8, a thermoelectrical cooler 3 for cooling the semiconductor laser 8, and an automatic laser temperature control circuit 2 for controlling a temperature of the semiconductor laser 8 by using the thermistor 4 and the thermoelectrical cooler 3. The optical transmitter further includes a monitor photodiode 9 for detecting emitted light of the semiconductor laser 8 and an automatic optical power control circuit 6 for feedback controlling the intensity of the emitted light by using the detection result.
In FIG. 2, an input signal a supplied from the input signal terminal 5 is supplied to the semiconductor laser driver circuit 1 to be an optical modulation current i.sub.m. The optical modulation current i.sub.m is superposed on a DC bias current I.sub.B to be supplied to the semiconductor laser 8 provided in a semiconductor laser diode module 7 through the impedance matching circuit 100 formed of a reflection preventing microstrip line and an impedance matching element 12. Thus, the semiconductor laser 8 emits light and its forward light output is guided to an optical fiber not shown while its backward light output is incident to the monitor photodiode 9. The impedance matching element 12 is connected to reduce a jitter in an emitted light signal and match impedances between the laser 8 and the driver circuit 1. The monitor photodiode 9 produces a current in response to the incident light and the current is converted into a voltage by means of a resistor 11 to be supplied to the automatic optical power control circuit 6. The circuit 6 controls the semiconductor laser driver circuit 1 in response to the voltage from the resistor 11 and sets the optical modulation current i.sub.m and the DC bias current I.sub.B to make constant an output current of the monitor photodiode 9. Thus, a power of the optical signal supplied to the optical fiber is made constant.
On the other hand, a temperature of the semiconductor laser 8 is detected by the thermistor 4 and the automatic laser temperature control circuit 2 controls the thermoelectrical cooler 3 in response to the detected output of the thermistor 4 to maintain constant the temperature of the semiconductor laser 8.
Further, in such a prior art, in order to drive the semiconductor laser by a high-frequency signal, it is necessary to reduce a parasitic inductance of the impedance matching circuit 100 and the impedance matching element 12. To this end, the impedance matching circuit comprises a so-called printed resistor formed on a board of ceramic by printing. In addition, as shown in FIG. 2, the semiconductor module 7 in which the semiconductor laser 8, the monitor photodiode 9 and the impedance matching element 12 (if necessary, further including the thermoelectrical cooler 3 and the thermistor 4) are hermetically sealed to be formed integrally is used in an optical transmitter.
The printed resistors used in the conventional optical transmitters as the impedance matching element 12 may have different resistance values for possible manufacturing errors ranging about plus or minus 10 percent even if they are manufactured in the same structure by the same manufacturing processes. Further, the semiconductor lasers used generally in the optical transmitters also may have different resistance values with deviations of about plus or minus 40% due to possible manufacturing errors in their cavity length or an amount of impurity to be doped. When the printed resistor and the semiconductor laser having such non-uniform resistance values and supplied as standard goods are directly used to manufacture the optical transmitters as they are, the impedances of the semiconductor laser and the impedance matching circuit result in their greatly differing and mismatching with each other.
Accordingly, although the printed resistor is provided as the impedance matching element 12, the impedance mismatching occurs actually between the semiconductor laser or the semiconductor laser module and the semiconductor driver circuit. Further, such a non-uniform impedance of the semiconductor laser for each product means that a waveform of the produced optical signal is non-uniform for each product.
When the semiconductor lasers and the printed resistors have different non-uniform impedances as described above, it is necessary to measure resistance values of the printed resistors and the semiconductor lasers one by one and select preferred printed resistors and the semiconductor lasers having the same resistance values as design values in order for the purpose of the impedance matching. Such a selection extremely reduces the yield of the printed resistors and the semiconductor lasers and increases a manufacturing cost of the optical transmitter.
In a conventional low transmission speed of about 100 Mb/s, the waveform of the signal has specifically no problem. Recently, however, it has been found that it is specifically important to use an optical transmitter capable of producing a signal having a fixed waveform in order to effect communication with a low bit error rate as an optical transmitter capable of operating at a very high transmission speed exceeding Gb/s is developed. However, the conventional optical transmitter including the impedance matching circuit is improved in reduction of the jitter in the optical signal but is not considered with respect to control of the signal waveform.